A. Generally
In communication networks such as cable television systems, it is often desirable to access information from individual subscriber terminals coupled to the network. For example, in a typical two-way cable television system, each subscriber is provided with an addressable converter that enables the selective reception of authorized television programs. Recent advances in the CATV industry have enabled the provision of pay-per-view services, wherein the cable television customer has an opportunity to select specific programs such as first-run movies. The information provided by the user station in this type of setup is limited.
However, other features available on CATV systems including shop-at-home, bank-at-home, viewer opinion polling, home multimedia PC applications and video conferencing require that the user station be more active in communicating with a communication control center. To be useful, a communications protocol selected for use with a CATV system wherein the user station's communication needs are variable desirably provides for efficient use of the available resources (e.g., data transmission bandwidth). Moreover, the system should desirably be upgradable if more bandwidth is available and/or the maximum bandwidth required by the user increases (e.g., HDTV).
Different applications require different bandwidth and delay requirements. Voice requires approximately 56-64 kilobits-per-second (kbps) for telephone quality but higher quality may require as much as 256 kbps. A video signal compressed using the MPEG-1 standard requires about 1.5 Megabits-per-second (Mbps) and other compression schemes that provide a higher quality video signal may require as much as 5-9 Mbps. HDTV quality requires approximately 20 Mbps.
Bandwidth requirements for data vary over a wide range. An application providing stock quotes may have a low bandwidth requirement (kbps) while the transfer of files over the network may require a much higher bandwidth.
In addition, while voice and video have stringent real-time delivery requirements, data usually does not. For example, a bandwidth of 1.5 Mbps can be provided by allowing the user station to send a packet of size 1.5 kilobits every millisecond or a packet of size 1.5 Megabits every second. While the latter may be acceptable in the case of a file transfer application, it would not be acceptable for a video conference where smaller packets have to be sent more frequently.
In an interactive environment, a user's bandwidth requirement may vary during a connection. For example, a user could be in the middle of a voice call and may want to add video to the call. This could cause his bandwidth requirement to go up, for example, from 250 kbps to 5 Mbps which, in turn, may cause the user station to change from sending 5 kilobit packets every 20 ms to sending 20 kilobit packets every 4 ms.
As mentioned, a solution to efficiently sharing available bandwidth needs to take into consideration the potential of users with varying bandwidth requirements. Current methods such as systems employing frequency division multiple access (FDMA) or time division multiple access (TDMA) can only allocate in multiples of a predetermined minimum frequency band or time slot, respectively. Moreover, TDMA requires all the users to have time synchronization, which adds complexity to the system. This is inefficient in the case of applications with variable bandwidth requirements.
B. Background Regarding CATV Topology
FIG. 1 shows a typical CATV system comprising at least one headend 102 connected to a plurality of hubs 104a-d interconnected in a star-like topology. The connections between hubs 104a-d are provided for redundancy purposes. The above connections have been made either by coaxial cables (coax), microwave links or optical fibers, although optical fibers are expected to be the medium of the future.
Emanating from each hub 104a are trunks 106 (typically 40-50) which serve a group of homes. Although current trunks are typically coaxial cables, they too are being replaced by optical fibers. Optical fiber trunks terminate at an optical node 108 which receives the optical signal, converts it to a coax compatible signal and transmits it to the homes.
Replacing much of the interconnections with optical fibers will likely increase the bandwidth available on the coax from approximately 300-400 MHz to 1 GHz. This will enable the provision of a few hundred digital PPV channels and interactive services like video-conferencing and home shopping in addition to the usual cable channels.